This section provides background information related to the present disclosure which is not necessarily prior art.
The air conditioning portion of an HVAC system includes numerous components. These components may include, for example, a compressor for compressing refrigerant in the HVAC system and a condenser for cooling the compressed refrigerant by heat exchange to condense the refrigerant to a liquid. Both the compressor and the condenser include a motor. The compressor uses the compressor motor to compress the refrigerant and the condenser uses the condenser motor to drive a fan for moving air across condenser coils for increased heat exchange. Additionally, or alternatively, the system may be operated in reverse as a heat pump to provide heating rather than cooling.
Compressors typically compress refrigerant to very high pressures. When a problem occurs with a compressor, such as a failure of the compressor or a power failure, the compressor motor may cease operating. If the compressor is, e.g, a scroll or screw compressor, the high pressure of the refrigerant may force the compressor motor to rotate in reverse, sometimes referred to as backspinning.
Many compressors employ permanent magnet compressor motors (i.e., motors having surface and/or embedded permanent magnets). A backspinning permanent magnet motor becomes a generator. Because of the high pressure of the refrigerant, a permanent magnet compressor motor may backspin at a high rate of speed and may, accordingly, generate a relatively high voltage. This voltage may exceed the voltage used to drive the compressor motor by a significant amount. For example, the magnitude of the generated voltage in some instances may be twice the magnitude of the compressor motor supply voltage. Such high voltage being generated by the compressor motor can damage electrical components in the HVAC system. Therefore, mufflers and check valves are commonly used in scroll compressors employing permanent magnet motors to inhibit reverse refrigerant gas flow and thereby inhibit backspin. Further, chopper resistors are commonly used with scroll compressors employing permanent magnet motors to produce a braking torque that inhibits reverse rotation (i.e., backspinning) of the compressor motor.